I think I know...

it was done for accessibility, and somebody forget to update styles. I talk about that: https://getbootstrap.com/docs/5.0/helpers/visually-hidden/

Solution, add that custom CSS:

.visually-hidden, .visually-hidden-focusable:not(:focus):not(:focus-within) {
  position: absolute !important;
  width: 1px !important;
  height: 1px !important;
  padding: 0 !important;
  margin: -1px !important;
  overflow: hidden !important;
  clip: rect(0,0,0,0) !important;
  white-space: nowrap !important;
  border: 0 !important;
}